1. Field of the Invention
The present invention relates generally to sensing of physical phenomena, particularly to optical image sensing, and more particularly to high dynamic range optical imaging for digital electronic night vision.
2. Description of the Related Art
Night vision capability provides warfighters a significant force multiplication benefit. Military operations are increasingly being conducted in urban terrain, which is much more demanding of dynamic range than traditional rural venues. In particular, soldiers conducting military operations during night in urban terrain may encounter light conditions ranging from “overcast starlight” to relatively bright artificial lighting. The dynamic range within a single scene may vary by 6 or 7 orders of magnitude or more, ranging from street lights (approximately 10 lux) to dark alleys (approximately 1 microlux). Present night vision systems use a single user adjustable exposure control to set a nominal range for present light conditions with a dynamic range that is generally limited to about 16 bits of sample resolution. What is needed is an ability to automatically capture a wider range of night vision scene information including dark areas within relatively bright scenes that can then be intensity scaled and represented to a soldier on a display.
Referring now to FIG. 1, a simplified schematic of a basic digital night vision pixel is illustrated, designated generally as 2. The digital night vision pixel 2 includes a sensor 3 that senses physical phenomena such as light. A typical light sensor is a P-N junction that generates photocurrent in proportion to the intensity of the light that impinges on the P-N junction. The sensor 3 is connected to a storage capacitor 4 that accumulates photocurrent generated by sensor 3 through an integration switch 5. The integration switch 5 controls an integration time of the sensor 3 which determines the effective sensitivity of the sensor 3. A reset switch 6 is connected to the storage capacitor 4 in parallel. The reset switch 6 is used to reset the sensor 3 to a state corresponding to no charge. A multiplexer (Mux) switch 7 connects the integration switch 5 to a shared A/D converter 8 that converts the analog signal to a digital signal. The resolution of the A/D converter 8 determines the dynamic range within a scene. A 16 bit A/D converter is about the limit; however, 20 or more bit resolution is needed for the previously described urban terrain application.
One prior art attempt to extend the dynamic range of an optical sensor is described by Tsutomu Nakamura and Kuniaki Saitoh in a paper entitled “Recent Progress in CMD Imaging” presented at the 1997 IEEE Workshop on Charge-Coupled Devices and Advanced Image Sensors. This paper describes a mode of operation of an image sensor whereby the sensor is scanned twice per field. The first scan outputs a signal with a short integration time in non-destructive readout mode; the second scan outputs a signal with a long integration time. The two signals are combined to generate an image with higher dynamic range. A disadvantage of this approach is that an entire scan of the field for each different integration time is needed, which limits the potential dynamic range that may be achieved. A further disadvantage of this approach is the potential for image degradation due to changes and movements in the scene between scans.
Another attempt to extend the dynamic range of an optical image sensor is described in U.S. Pat. No. 6,369,737, issued to Yang et al, entitled “Method and Apparatus for Converting a Low Dynamic Range Signal to a Large Dynamic Range Floating-Point Digital Representation”. Yang et al disclose an optical image sensing apparatus with a sub-converting A/D converter embedded in the image sensing array to provide a digital sample word wider than the resolution of the A/D converter. The resolution of the rather limited subconverting A/D is only moderately extended by that approach.
Yet another prior art attempt to extend the dynamic range of an optical image sensor is described in U.S. Pat. No. 5,614,948 issued to Hannah, entitled “Camera Having an Adaptive Gain Control”. In the Hannah device, the gain of an output amplifier is varied over different regions of the imaging array as indicated by the row and column addresses of the array. While this approach can reduce the range of signal excursions on the analog output path, it has no effect on the dynamic range of the sensing element of the array.
What is needed is an optical image sensor with the ability to sense and capture images of a single scene with light intensity conditions varying by 20 bits or more. What is further needed is a high dynamic range optical image sensor that is capable of rapidly capturing an indefinitely long series of images of scenes, each with light intensity conditions varying by 20 bits or more.